1. Field of the Invention
The present invention relates generally to a packet service communication system, and in particular, to an apparatus and method for transmitting data using transmit antenna diversity in a packet service communication system.
2. Description of the Related Art
In general, a packet service communication system is designed to transmit a large volume of burst packet data to a plurality of user equipments (UEs). In particular, HSDPA (High Speed Downlink Packet Access) was proposed as a packet service communication system suitable for transmission of a large volume of data at a high rate.
HSDPA is a generic term referring to devices, systems and methods using a HS-DSCH (High Speed-Downlink Shared CHannel) for supporting downlink packet data transmission at a high rate and its related control channels in a W-CDMA (Wideband-Code Division Multiple Access System). For simplicity, HSDPA, which was proposed by the 3GPP (3rd Generation Partnership Project) and adopted as the standard for 3rd generation asynchronous mobile communication systems, will be described by way of example. It is to be appreciated that the present invention is also applicable to any other system implementing transmit antenna diversity through two or more transmit antennas.
Three techniques have been introduced into the HSDPA communication system to support high-speed packet data transmission: AMC (Adaptive Modulation and Coding), HARQ (Hybrid Automatic Retransmission Request), and FCS (Fast Cell Select). These techniques are described hereinbelow:
The AMC technique provides a modulation scheme and a coding method, which are selected for a data channel according to the channel condition between a Node B and a UE, to thereby increase the use efficiency of the entire cell. Modulation schemes and codings are used in combination. Each modulation and coding combination is termed an MCS (Modulation and Coding Scheme). MCSs can be labeled with level 1 to level N. A data channel signal is modulated and encoded by an MCS adaptively chosen according to the channel condition between the UE and its communicating Node B. Thus, the system efficiency of the Node B is increased.
In accordance with a typical ARQ (Automatic Retransmission Request), ACK (Acknowledgement) signals and retransmission packet data are exchanged between a UE and an RNC (Radio Network Controller). Meanwhile, the HARQ scheme, especially an N-channel SAW HARQ (N-channel Stop And Wait HARQ), adopts the following two novel procedures to increase ARQ transmission efficiency. One is to exchange a retransmission request and its related response between a UE and a Node B, and the other is to temporarily store bad data and combine the stored data with a retransmission version of the data. In the HSDPA communication system, ACK signals and retransmission packet data are exchanged between the UE and the MAC (Medium Access Control) HS-DSCH of the Node B, and the N-channel SAW HARQ establishes N logical channels and transmits a plurality of packets without receiving an ACK signal for a previous transmitted packet. As compared to the N-channel SAW HARQ technique, the SAW ARQ technique requires reception of an ACK signal for a previous transmitted packet data to transmit the next packet data. Thus the ACK signal must be awaited for the previous packet despite the capability of transmitting the current packet data. On the contrary, the N-channel SAW HARQ allows transmission of successive packets without receiving the ACK signal for the previous packet data, resulting in the increase of the channel use efficiency. That is, N logical channels, which can be identified by their assigned times or channel numbers, are established between the UE and the Node B, so that the UE can decide the channel that has delivered a received packet and take an appropriate measure such as rearrangement of packets in the right order or soft combining of corresponding packet data.
In the FCS technique, when a UE supporting HSDPA is positioned in a soft handover region, it fast selects a cell in a good channel condition. Specifically, if the UE enters a soft handover region between a first Node B and a second Node B, it establishes radio links with a plurality of Node Bs. A set of Node Bs with which the radio links are established are called an active set. The UE receives HSDPA packet data only from the cell in the best channel condition, thus reducing the whole interference. The UE also monitors channels from the active Node Bs periodically. In the presence of a cell better than the current best cell, the UE transmits a best cell indicator (BCI) to all the active Node Bs to substitute-the new best cell for the old best cell. The BCI includes the ID of the new best cell. The active Node Bs check the cell ID included in the received BCI and only the new best cell transmits packet data to the UE on the HS-DSCH.
As described above, many novel techniques have been proposed in order to increase data rate in the HSDPA communication system. The data rate increase is a dominant factor determining performance in 1×EV-DO (Evolution-Data Only) and 1×EV-DV (Evolution-Data and Voice) as well as HSDPA. Aside from AMC, HARQ, and FCS, a multiple antenna scheme is used as a way to increase data rate. Since the multiple antenna scheme is performed in the space domain, it overcomes the problem of limited bandwidth resources in the frequency domain. The multiple antenna scheme is realized usually by nulling, which will be described in detail later.
Before undertaking the description of the multiple antenna scheme, multi-user diversity scheduling will first be described. A packet service communication system such as HSDPA decides the states of a plurality of user channels requesting packet service based on their feedback information and transmits packet data on a user channel having the best channel quality. The resulting SNR (Signal-to-Noise Ratio) gain increase effects diversity. A diversity order representing a diversity gain corresponds to the number of users requesting packet service at the same time.
Under a radio channel environment, a mobile communication system suffers signal distortion because of various factors such as multi-path interference, shadowing, propagation attenuation, time-varying noise, and interference. Fading caused by multi-path interference is closely associated with the mobility of a reflective object or a user, that is, the mobility of a UE. The fading results in mixed reception of an actual transmission signal and an interference signal. The received signal is eventually a transmission signal involving serious distortion, which degrades the entire mobile communication system performance. Fading is a serious obstacle to high-speed data communication in a radio channel environment in that the fading incurs distortion in the amplitude and phase of a received signal. In this context, transmit antenna diversity, which is a type of multiple antenna scheme, has emerged as an effective way to combat fading.
Transmit antenna diversity seeks to minimize fading-caused data loss and thus increase data rate by transmitting a signal through at least two antennas.
Transmit antenna diversity is classified into time diversity, frequency diversity, multi-path diversity, and space diversity.
Space diversity is used for a channel having a small delay spread, for example, an indoor channel and a pedestrian channel being a slow fading Doppler channel. The space diversity scheme achieves diversity gain by use of two or more antennas. If a signal transmitted through one antenna is attenuated by fading, diversity gain is obtained by receiving signals transmitted through the other antennas. Space diversity is further branched into receive antenna diversity using a plurality of receive antennas and transmit antenna diversity using a plurality of transmit antennas.
Frequency diversity achieves diversity gain from signals transmitted with different frequencies and propagated in different paths. In this multi-path diversity scheme, the multi-path signals have different fading characteristics. Therefore, diversity is obtained by separating the multi-path signals from each other.
The transmit antenna diversity scheme is implemented in a closed loop or an open loop. The closed loop transmit antenna diversity differs from the open loop one in that a UE feeds back downlink channel information to a Node B in the former, while the feedback information is not required in the latter. For the feedback, the Node B transmits a different pilot signal through each transmit antenna. The UE measures the phase and power of the received pilot channel for each transmit antenna and selects an optimum weight based on the phase and power measurements.
The mobile communication system must overcome fading that seriously influences communication performance in order to carry out high-speed data transmission reliably. This is because fading reduces the amplitude of a received signal by several decibels to tens of decibels. Hence, the above-described diversity schemes are adopted to combat fading. For example, a CDMA communication system uses a rake receiver for implementing diversity reception based on the delay spread of a channel. Besides the above-described methods, the data rate can be increased by carrying out coherent transmission utilizing the characteristics of a space channel. Thus, SNR increases in proportion to the number of antennas.
Meanwhile, antenna beamforming increases limited system transmission capacity in the packet service communication system. The antenna beamforming is signal transmission from a plurality of directional antennas. To prevent a signal transmitted through one antenna from interfering with a signal transmitted through another antenna, nulling is used. The nulling technique can increase the volume of transmitted packet data only if antenna beamforming is performed with antennas spaced by a predetermined gap. It is not feasible when the gap between antennas is rather wide. The antennas are spaced by a relatively short distance
  λ  2for antenna beamforming, whereas they are spaced by a relatively long distance 10λ for transmit antenna diversity. Since there are no correlations between antennas in terms of antenna distance, it is impossible to apply nulling for the transmit antenna diversity.
As described above, beamforming is a technique using nulling based on correlations between antennas spaced by a relatively short distance, for example,
      λ    2    .The nulling technique makes antenna weights w1Hh2=0 and w2Hh1=1 so that a first UE receives only its signal r1, not data d2 for a second UE and the second UE also receives only its data d2, not the signal r1 for the first UE. Here, w1 is a weight for the first UE and w2 is a weight for the second UE. h1 is a channel delivering the signal r1 and h2 is a channel delivering the signal r2. The mathematical expression of nulling is presented in Equation 1 as follows.
                                          W                          Mode              ⁢                              -                            ⁢              1                        H                    ⁢          W                =                  [                                                    2                                                              1                  +                  j                                                            0                                                              1                  -                  j                                                                                                      1                  -                  j                                                            2                                                              1                  +                  j                                                            0                                                                    0                                                              1                  -                  j                                                            2                                                              1                  +                  j                                                                                                      1                  +                  j                                                            0                                                              1                  -                  j                                                            2                                              ]                                    (        1        )            
If a channel condition is set in the manner that always generates weights satisfying the above condition, co-channel interference is completely eliminated and thus system capacity is in fact doubled. Nulling is always possible theoretically if the number of UEs to be nulled, including a desired UE, is less than that of the number of antennas by one. However, this ideal situation is possible only when the antennas are fully correlated and differ from each other only in phase. Therefore, the beamforming nulling technique is very difficult to realize in a radio channel environment for mobile communication.
As compared to beamforming, there are little correlations between antennas spaced by a relatively long distance, for example, 10λ in the multiple antenna system. Hence, the nulling technique is not applicable, especially in the CDMA mobile communication system, because the number of antennas exceeds that of UEs being serviced at the same time, and thus exceeds the number of degrees of freedom to set specific signal processing weight values for nulling (i.e., number of antennas−1).
To realize transmit antenna diversity, a transmit antenna array (TxAA) is used. A TxAA is operated in a first TxAA mode (TxAA mode 1) or a second TxAA mode (TxAA mode 2). In TxAA mode 1, UEs calculate weights w1 and w2 maximizing signal reception power using pilot signals received from a Node B. The UEs then deliver the weights w1 and w2 to the Node B on a particular channel, for example, in an FBI (FeedBack Information) field of a DPCCH (Dedicated Physical Control Channel). Four weights 00, 01, 10 and 11 are available to the UEs in TxAA mode 1. As compared to TxAA mode 1, all power information including phase and amplitude is controlled in TxAA mode 2. While TxAA mode 1 addresses only phase, TxAA mode 2 additionally controls amplitude. A total of 16 weights are defined which represent phases and amplitudes separately.
A weight w is related to a transmit antenna array channel h, as w=h (w and h are vectors). An FDD (Frequency Division Duplex) mobile communication system requires a UE to feedback transmission channel information to a Node B so that the Node B is informed of a transmission channel because the characteristics of a transmission channel and a reception channel are different. To do so, the UE computes a weight and feeds the weight information back to the Node B in the channel h in TxAA mode 1 or TxAA mode 2. In TxAA mode 1, only a phase component is quantized in two bits in the weight information w=[1w11 exp(jθ1), 1w21exp(jθ2)], and fed back to the Node B. Therefore, the phase accuracy is π/2 and the quantization error is up to π/4. To increase the efficiency of the feedback, one of the two bits is refined by updating it each time. For example, 2-bit combinations {b(2k), b(2k−1)} and {b(2k), b(2k+1)} are available. Here, b is a bit feedback on a slot basis each time. In TxAA mode 2, the components of the weight information, both phase and amplitude, are fed back. The phase is 3 bits and the amplitude is 1 bit. Hence, the phase accuracy is π/4 and the quantization error is up to π/8. To increase the efficiency of the feedback, one of the four bits is refined by updating it each time in a progressive refinement mode. While each bit is an orthogonal basis in a refinement mode, there is no such regulation in the progressive refinement mode.
In view of the nature of the HSDPA communication system, packet data is transmitted on a predetermined unit basis, for example in frames, to a UE in the best channel condition. Channel quality information is received from a plurality of UEs requesting HSDPA service and their channel conditions are decided based on the channel quality information. The UE in the best channel condition is selected and packet data is delivered only to the selected UE at a corresponding point in time. Therefore, even if system transmission resources are available to more UEs, only the selected UE receives the HSDPA service. As a result, the efficiency of the transmission resources is reduced.